The basic premise is kind of similar to my blyrics application, narrowing down the subset of possible answers in a search query. If you remember a small section of lyrics from a song (maybe just a few words) and try to google it you may end up with way too many results. If you restrict your search to just the subset of songs that you have listened to, its far easier to sift through the results.
I want to apply that idea to movies. Initially I just wanted an offline search tool for my own movie collection so I could search by actors, directors, genres, arbitrary tags that I add, etc. I also want to track the number of times the movie has been played and the last time it was played. I'm not sure how exactly I want to utilize that data but at first it will just let me find movies I haven't seen in a while. Maybe eventually I could use the patterns in my watch history to suggest movies to watch.